Vibrio cholerae is a water-borne pathogen and causative agent of cholera. Its ability to disseminate and persist in fresh water establishes a reservoir from which outbreaks occur. However, the strong link between epidemics and overcrowding suggests a more efficient mode of fecal-oral transmission, which is supported by studies showing rapid and efficient spread of cholera within households. Knowledge of the bacterial and environmental factors that influence dissemination and transmission would contribute to a better understanding of cholera outbreaks, and to better public health measures to prevent them. We showed that V. cholerae exits humans in a physiological state(s) primed for dissemination and transmission. However, we found that the V. cholerae genes that contribute to dissemination and transmission are largely distinct, and that expression of individual genes occurs in only a fraction of the cells being shed. Thus, we hypothesize that distinct subpopulations are shed, one primed for dissemination and the other for rapid transmission. In Aim 1, we will demonstrate and characterize these different subpopulations by physically separating them for phenotypic and gene expression studies. In clinical samples, we identified a lytic phage that closely associates with epidemic V. cholerae. We discovered a dynamic arms race between this phage and V. cholerae that is occurring within humans and aquatic environment. In Aim 2, we will determine how dynamics of phage predation impact dissemination and transmission of V. cholerae. We found that vaccination with purified V. cholerae outer membrane vesicles elicits antibodies recognizing LPS O-antigen, that these antibodies are sufficient for protection, and that V. cholerae passing through an immune host are rendered avirulent. In Aim 3, we will examine the mechanism of protection and blocking of transmission. These studies will establish a basis for understanding how V. cholerae simultaneously promotes dissemination and transmission, how pervasive lytic phages impact these processes, and how vaccination blocks infection and transmission. This knowledge will enhance understanding of water- borne pathogens and help pave the way to development of interventions that target the transmissible and disseminative forms of such pathogens.